Abstract: Good air tightness of filter membranes is necessary for opening excavation chamber under high pressure. To improve the value of air tightness, one should understand the pore structure and permeability change of filter membranes during chamber opening under high pressure. Taking the sandy cobble stratum of Nanjing Weisan Road river-crossing tunnel as an example, the air tightness tests are carried out for filter membranes formed under different pressures by a self-developed airtightness apparatus. Its pore structures and measured diameters are observed, and the changes of permeability coefficient are analyzed. The results show that the thickness of the filter membranes is 5.0 mm under 0.2 MPa for 6 hours, the porosity is about 67%, the filter membranes are compressed during the air tightness tests and at the first stage it is the largest. With the growth of the air pressure, the large pore group is preferentially compressed. The basic unit of filter membranes is a skeleton-like structure, and the pore structure is mainly inter-particle pore and overhead pore; furthermore, the pore is not continuous and poorly connective. The pore size has a broader distribution, and the group of 0.1~3.0 μm has an absolute advantage. The permeability coefficient in this experiment has a magnitude of 10^-9 cm/s. The results define the change of the filter membranes during air tightness, and it plays a significant role in improving the value of filter membranes during air tightness.